Despite its namesake, did you know that mean solar time was primarily introduced when mechanical clocks and other man-made time keeping devices of considerable accuracy gained widespread use?
By: Ringo Bones
The introduction of clocks and watches toward the end of the 17th Century forced the introduction of mean solar time. Despite its rather misleading name suggesting that one’s accuracy is dependent on the other, the mean solar time is defined by the “mean sun” – defined, descriptively, as a fictitious body which moves in the celestial equator at the same average speed of the true sun. The hour angle of the mean sun is the mean solar time by qualitative – but not rigorous astronomical – definition. The difference in time between true solar time and mean solar time is called the “equation of time.” It is not really an equation, but only the difference in time; it can be as large as 16 minutes.
As mentioned previously, mean solar time is obtained by observing the stars and not the sun. Star observations furnish “sidereal time” as sidereal is derived from the Latin word sidereus meaning star or constellation. By means of a mathematical formula, sidereal time is converted to mean solar time. Mean solar time is rigorously defined in such a way that it is a strict measure of the angular position of the earth about its axis, irrespective of whether or not there are changes in the speed of rotation. It is because mean solar time is a measure of the rotation of the earth that it is needed for the determination of longitude and for any other application where the angular position of the earth in space must be known.